Adhesive tape

ABSTRACT

An adhesive tape embodying a core strip (preferably a metallic strip) and an adhesive upon the major surfaces of the strip. The cross section of the core has a substantially thick portion that diminishes in thickness towards the edges. In a preferred embodiment the edges are provided with fins that prevent the adhesive from being squeezed from between structural parts joined by the tape as the parts are drawn together by rivets, bolts, or the like.

Unite States atent Orowan [15] 3,655,424 1451 Apr. 11, 1972 ADHESIVETAPE [72] Inventor: Egon Orowan, Belmont, Mass.

[73] Assignee: Massachusetts Institute of Technology,

Cambridge, Mass.

[22] Filed: May 24, 1968 [21] Appl. No.: 731,769

117/127,128.4, 27, 31; 85/50 AT, 1 JP; 151/38; 29/458; 161/213-2251,793,116 2/1931 Monroe ..151/38 2,535,852 12/1950 Hatfield ....1 17/685UX 2,650,185 8/1953 Larson et a1. ..1 17/685 X 3,066,060 11/1962 Gross161/215 3,458,132 8/1969 Newton ..151/38 X Primary Examiner-William D.Martin Assistant ExaminerBernard D. Pianalto Attorney-Thomas Cooch,Martin M. Santa and Robert Shaw [5 7] ABSTRACT An adhesive tapeembodying a core strip (preferably a metallic strip) and an adhesiveupon the major surfaces of the strip. The cross section of the core hasa substantially thick portion that diminishes in thickness towards theedges. In a preferred embodiment the edges are provided with fins thatprevent the [56] References Cited adhesive from being squeezed frombetween structural parts UNITED STATES PATENTS joined by the tape as theparts are drawn together by rivets,

bolts, or the like. 2,601,336 6/1952 Johannsen ..117/122 UX 3,271,2329/1966 Moore ..161/175 17 Claims, 7 Drawing Figures 5' 7\ Y '7 7 T77 I7T7?- 7 I 7 52 1' W l i 5 PATENTEDAPR 1 1 I972 III!!! PRIOR ART /////&

PRIOR ART ""II--, 'l ll II II II II II II II II II II II I V" T i/k/qw/INVENTOR EGON OROWAN TTORNEY ADHESIVE TAPE The invention hereindescribed was made in the course of work performed under a contract withthe Air Force Ofiice of Scientific Research.

The present invention relates to adhesive tapes that embody a carrierstrip and an adhesive and are particularly useful in connection withriveted and pinned joints.

Adhesive bonding has been successfully used in, for example, aircraftmanufacture for joining metallic parts. In joining structurallyimportant parts of the aircraft, designers are reluctant to entrust thebond to adhesion alone, mainly because the accidental presence of dirtmay cause local weakness that cannot be recognized by nondestructivetesting. It would be desirable, however, to use adhesives together withrivets or pins in such a manner as to eliminate a well known shortcomingof the latter. I-lot riveting in steel structures prevents sliding ofthe joined parts approximately up to the load at which the rivet wouldsuffer shear fracture. Cold riveted joints in light alloy structures,however, are not fully immobilized by friction; and slight relativemovement between the members under cyclic loading results in fretting atthe abutting surfaces of the joint and at the rivet holes, therebyproducing, eventually, fatigue cracks. The practice has often been,therefore, to introduce between the plates of a lap joint adhesivematerial as a thin uniform layer, with or without a carrier material (asscrim cloth); the use of the adhesive in this fashion contributes torelieve the load on the rivets to a relatively small extent and givessome protection against fretting between the parts joined. It does not,however, prevent fretting at the rivet holes, since the adhesive has avery low shear modulus compared to the metals usually joined in thismanner and, therefore, cannot immobilize the joint sufficiently toprevent rubbing at the rivet holes. The relative motion of the joinedparts can be reduced by making the adhesive layer very thin; in thiscase, however, the advantage of distributing the load over the bondedsurface by means of the highly elastic adhesive layer is lost, and theadhesive joint becomes weak relative to its surface area. This wellknown property of lap joints is often remedied by the use of scarfing,i.e., the gradual thinning of the thickness of the joined plates towardstheir margins. However, in aircraft scarfing is usually not practicable,and it would be difficult to make the adhesive layer thin enough toprovide a joint of high shear rigidity.

Accordingly, an object of the invention is to provide an adhesive tapefor joining structural parts in which the adhesive layer varies from avery small interior thickness, to provide high rigidity, to a greaterthickness near the margins of the adhesive layer, to allow a gradualincrease of the shear stress transmitted by the layer from its marginsinward and thereby provide the effect achieved by scarfing, aspreviously discussed.

Another object is to provide an adhesive tape particularly adapted foruse in connection with rivets, pins, bolts, or similar fastening devicesfor reducing the displacements that the joint would suffer in theabsence of the adhesive and thereby relieve the fastening devices of agreater part of the load to which they would otherwise be exposedwithout the adhesive or with conventional adhesives of approximatelyconstant thickness.

Other and further objects will be apparent in the discussion to followand will be particularly delineated in the appended claims.

By way of summary, the objects of the invention are attained, generally,in an adhesive tape for joining metal or similar structural parts, thatcomprises, a solid core strip which acts as a carrier for an adhesive.The core strip, which is preferably made of metal, has a Youngs modulusat least a substantial fraction of the Youngs modulus of the structuralparts to be joined and a cross section that diminishes from asubstantially thick portion towards at least one edge of the strip. Anadhesive layer is disposed on the core strip.

The invention will be described with reference to the accompanyingdrawing in which:

FIG. 1 is a side sectional view of two plates connected together at ariveted lap joint with an adhesive disposed between the plates at thejoint and represents one prior art means for effecting a bond betweenplates;

FIG. 2 is a view similar to FIG. 1, except that the plates shown arescarfed in the region of the joint, and discloses a prior art means moreuseful in wood-to-wood joints than in metal-to-metal joints;

FIG. 3 is a side sectional view of two plates connected together at ariveted lap joint with an adhesive tape embodying the present inventiveconcept disposed between the plates;

FIG. 4 illustrates a modification of the embodiment of FIG.

FIG. 5 is an isometric view, on reduced scale, of the adhesive tapeshown in end view in FIG. 4;

FIG. 6 is an end view of a modification of the tape of FIG. 3; and

FIG. 7 is an end view of a further modification.

Turning now to the drawing, an adhesive tape is shown at 1 in FIG. 3 forjoining structural parts, as the plates shown at 2 and 3. The tape 1comprises a solid core strip 4 having a cross section that diminishes inthickness from a substantially thick internal or central portion 5towards the margins 6 and 7 thereof. The Youngs modulus of the strip 4is preferably at least a substantial fraction of the Youngs modulus ofthe structural parts to be joined (but may be greater), and when theplates 2 and 3 are of a metal, the strip 4 is preferably made also ofmetal. An adhesive layer 8 is disposed upon each major surface of thecore strip 4. The plates 2 and 3 shown are secured together by rivetsand also by the adhesive 8. The numeral 4 is used throughout todesignate the core strip although, as will be evident hereinafter, thecross section shape of the strip may differ between the illustrativeexamples.

It is in order, now, to discuss the forces acting at the joint in FIG. 3as a result, for example, of tension forces F and F in the x directionupon the plates 2 and 3. If a tensile force acts upon the joint shown inFIG. 3, it produces two kinds of displacement between the free end ofone plate (as the free end 2 of the plate 2) and the opposite point(designated 3) of the other plate. First, there is a tangentialdisplacement in the direction of the tensile force (x direction),arising from the circumstance that the full force acts in the plate atthe region 3 where it faces the free end 2 of the opposite plate, but noforce whatever acts at the free end 2'. Second, the tensile force has atendency to rotate the rivets (as the rivet 9) counterclockwise whilethe joined margins of the two plates tend to bend so as to move apart atthe free ends of both plates, i.e., the free end 2' moves in the ydirection relative to the region 3' by virtue of the counterclockwiserotative movement. Since the elastic modulus of the adhesive is muchlower than that of the metal plates, the tangential displacement at themargins as well as the amount by which the gap between the plates tendsto open up at the margin is not influenced greatly by the adhesive. Ifthe displacements are more or less fixed for a given load, the adhesivelayer at the margin suffers a shear strain from the tangentialdisplacement that is inversely proportional to the thickness of thelayer. Similarly, the layer suffers a tensile strain in the directionnormal to the plates (in the y direction) which is also inverselyproportional to its thickness. Since the adhesive tears in shearfracture at some critical magnitude of the shear strain, and in tensilefracture at a critical value of the tensile strain, or it peels off themetal at critical values of the tensile and shear strains, its fractureor peeling is prevented if its thickness in the critical marginalregions is great enough. On the other hand, if the entire layer isthick, it suffers a large uniform shear strain which imposes upon therivets and holes pressures even higher than they would be exposed towithout any adhesive. According to the present invention, however, theadhesive layer is very thin in the interior of the joined area, inparticular around the rivets, to reduce the shear elasticity, but itsthickness in the marginal parts of the joined area is large enough toprevent dangerously high shear and tensile stresses and strains.

As the plates 2 and 3 are pressed together by the rivets 9 in theformation of the joint, there is a tendency for the adhesive 8 to besqueezed out from between the plates; this squeezing out has beenretarded hitherto by embedding a fabric of glass fibers or polymerfibers or the like in the adhesive. Also moisture and air can enter thejoint to attack the adhesive elements. To prevent either event it isproposed to provide marginal fins as shown at 14 and 15 in FIGS. 4, 5,and 6 and at 16 and 17 in FIG. 7, projecting out of the plane of thestrip 4. The adhesive, in this instance, is disposed within recesses 18formed on either side of the thick portion and between the thick portionand the margins where the strip thickness is increased abruptly to formsaid fins. The fins l4 and are shown to taper to thin lamina 20, 21, and22, 23, respectively. The fins should be thin and highly elastic toprevent the possibility of fretting of the plates at the points ofcontact with the fins under alternating loads. In spite of the thinness,the fins should be effective to prevent extrusion of the adhesive duringjoining; thus, it is of advantage to curve the fins back towards theinterior of the core strip as is shown in FIGS. 4, 5, and 6, or they maybe curved inwardly as shown in FIG. 7 where the retrotlexed part of thefin is large enough to form a partially closed channel, the two outersurfaces of which (as the surfaces designated 16 and 16" in FIG. 7)adhere to structural parts joined strongly enough to counteractseparation of the plates at the free ends thereof under load, asdescribed previously herein.

The strip 4 may be formed by rolling which may be followed by bending orotherwise forming the fins l4 and 15. The retroflexed fins I6 and 17 inFIG. 7 may be formed by bending the edges of the strip around an axisparallel to the axis of the strip, and, again, recesses 18 are formedbetween the tape in' terior and the margins, thereby to retain theadhesive, as before discussed. And, as shown at 24 (and 24 in FIG. 5),the adhesive may completely cover the strip 4, the adhesive at 24 (or24') being forced towards the margins as the joint is formed as shown,for example, in FIG. 4.

In the embodiment of FIG. 6 the core strip 4 contains a plurality ofsubstantially thick portions 25 and 26 alternating transversely withportions of reduced thickness 28, 30, and 31 forming recesses, as therecess designated 32. The rivets 9 in a completed joint are placed withtheir axes along the lines labeled B-B, and CC in FIG. 6 and along theline labeled A--A in FIG. 7.

The strip 4 may be fabricated of aluminum alloy, titanium, steel, orother metal or non-metal having the required Youngs modulus. If theplates 2 and 3 are a particular metal as, for example, titanium, thestrip 4 is preferably made of the same metal to avoid stresses bydifferential thermal expansion or contraction. The adhesive layer 8 maycontain a fabric of glass fiber, polymer fiber, or metal fiber, it beingkept in mind that the fabrics should be restricted to the recesses andshould not be present in the regions 24 and 24 at the face of the strip4 over the thick portion 5 where the adhesive layer must be thin for thereasons previously given, unless a very thin fabric tape is used. Inaddition, the adhesive may contain a filler such as metal powder or thelike to increase its rigidity.

In order to show features of the invention distinctly, the thickness ofthe strip relative to its width is exaggerated in FIGS. 3, 4, 5, 6 and7. In an actual tape the thickness shown would be reduced to roughlyone-half to one-fourth the sizes shown. Also, the shank diameters of therivets and the thickness of the plates would vary somewhat from theillustra' tive examples in a manner well known to those skilled in theart.

In order to provide a strip 4 slightly more compressible than the solidstrip shown in the figures, it may have one or more very flat shapedchannels in its interior extending along the length of the strip.

The term strip" is used herein in its usual sense to denote a body whosewidth dimension (W) is much greater than its thickness (T) (W/T--5) andwhich is produced in lengths I w Modifications of the invention hereindescribed will occur to those skilled in the art and all suchmodifications are deemed to be within the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:

1. An adhesive tape for joining structural parts that comprises, incombination, a core strip having a thickness dimension much less thanits width dimension acting as a carrier for an adhesive, said core striphaving a Youngs modulus at least a substantial fraction of the Youngsmodulus of the structural parts to be joined and a cross section thatdiminishes from a substantially thick portion towards at least onemargin of the strip, and an adhesive layer disposed upon both faces ofthe core strip, said core strip having a cross sectional shape whichprovides in the space between the parts joined a gap for the adhesivediminishing in thickness from at least one margin toward the interior ofthe strip, thereby to provide a thin adhesive layer at said thickportion of the core strip and a thicker adhesive layer at said onemargin.

2. An adhesive tape as claimed in claim 1, in which the margin of thestrip is provided with at least one thin fin projecting out of the planeof the strip to form at least one recess, at least some of the adhesivebeing disposed within the recess in sufficient amount to fill the gapbetween the core strip and the structural parts joined.

3. An adhesive strip as claimed in claim 2, in which the margin of thestrip is provided with two thin fins projecting outward from oppositefaces of the strip to form recesses, at least some of the adhesive beingdisposed within the recesses in sufficient amount to fill the gapbetween the core strip and the structural parts joined.

4. An adhesive tape for joining structural parts that comprises, incombination, a core strip having a thickness dimension much less thanits width dimension acting as a carrier for an adhesive, said core striphaving a Youngs modulus at least a substantial fraction of the Youngsmodulus of the structural parts to be joined and a cross section thatdiminishes from a substantially thick portion towards both margins ofthe strip and a layer of adhesive disposed upon both faces of the corestrip, the composite tape comprising the core strip and the adhesivebeing of substantially uniform thickness between the margins.

5. An adhesive tape as claimed in claim 4, said adhesive containing afiller of metal particles.

6. An adhesive tape as claimed in claim 4, in which each margin of thestrip is provided with a thin fin projection out of the plane of thestrip to form a recess, at least some of the adhesive being disposedwithin the recess in sufficient amount to fill the gap between the corestrip and the structural parts joined, each fin being oriented to pointsubstantiaIly inwardly towards the interior of the core strip.

7. An adhesive tape as claimed in claim 6 and in which said at least onefin is curved inwardly towards the interior of the core strip to presenttwo substantial surfaces each adhering to one of the structural partsjoined.

8. An adhesive tape as claimed in claim 4 in which the core strip ismetallic.

9. An adhesive tape as claimed in claim 4 in which the adhesive layercontains embedded reinforcing woven glass fiber.

10. An adhesive tape as claimed in claim 4 in which the adhesive layercontains embedded polymer fiber.

11. An adhesive tape as claimed in claim 4 in which the adhesive layercontains embedded metal fiber.

12. An adhesive tape as claimed in claim 4 in which the core strip has aplurality of substantially thick portions alternating with portions ofreduced thickness forming recesses.

13. An adhesive tape as claimed in claim 12 in which the thickness ofthe strip at said margins is increased abruptly to substantially thethickness of said thick portion to provide fins at the margins.

14. An adhesive tape as claimed in claim 4 in which the thickness of thecore strip at said margins is increased abruptly to substantially thethickness of said thick portion to provide fins at the margins.

15. An adhesive tape as claimed in claim 14 in which each margin of thestrip is provided with two thin fins projecting outward from oppositefaces of the strip to form recesses, at least some of the adhesive beingdisposed within the recesses in sufficient amount to fill the gapbetween the core strip and the structural parts joined.

16. An adhesive tape as claimed in claim 4 having fins at said margins,the fins being curved inwardly towards the interior of the core strip,the retroflexed part of each fin being large enough to form a partiallyclosed channel the two outer surfaces of which are adapted to adhere tothe structural parts joined.

17. An adhesive tape for joining structural parts that comprises, incombination, a core strip having a cross section of non-uniformthickness and having a maximum thickness dimension much less than thewidth dimension of the strip acting as a carrier for an adhesive, saidcore strip having a Youngs modulus at least a substantial fraction ofthe Young's modulus of the structural parts to be joined, and anadhesive layer disposed upon the both faces of the core strip, said corestrip having a cross sectional shape which provides in the space betweenthe joined parts a gap for adhesive that diminishes in thickness from atleast one margin of the strip toward the interior region thereof,thereby to provide a thin adhesive layer at said interior region of thecore strip and a thicker adhesive layer at said one margin.

2. An adhesive tape as claimed in claim 1, in which the margin of thestrip is provided with at least one thin fin projecting out of the planeof the strip to form at least one recess, at least some of the adhesivebeing disposed within the recess in sufficient amount to fill the gapbetween the core strip and the structural parts joined.
 3. An adhesivestrip as claimed in claim 2, in which the margin of the strip isprovided with two thin fins projecting outward from opposite faces ofthe strip to form recesses, at least some of the adhesive being disposedwithin the recesses in sufficient amount to fill the gap between thecore strip and the structural parts joined.
 4. An adhesive tape forjoining structural parts that comprises, in combination, a core striphaving a thickness dimension much less than its width dimension actingas a carrier for an adhesive, said core strip having a Young''s modulusat least a substantial fraction of the Young''s modulus of thestructural parts to be joined and a cross section that diminishes from asubstantially thick portion towards both margins of the strip and alayer of adhesive disposed upon both faces of the core strip, thecomposite tape comprising the core strip and the adhesive being ofsubstantially uniform thickness between the margins.
 5. An adhesive tapeas claimed in claim 4, said adhesive containing a filler of metalparticles.
 6. An adhesive tape as claimed in claim 4, in which eachmargin of the strip is provided with a thin fin projection out of theplane of the strip to form a recess, at least some of the adhesive beingdisposed within the recess in sufficient amount to fill the gap betweenthe core strip and the structural parts joined, each fin being orientedto point substantially inwardly towards the interior of the core strip.7. An adhesive tape as claimed in claim 6 and in which said at least onefin is curved inwardly towards the interior of the core strip to presenttwo substantial surfaces each adhering to one of the structural partsjoined.
 8. An adhesive tape as claimed in claim 4 in which the corestrip is metallic.
 9. An adhesive tape as claimed in claim 4 in whichthe adhesive layer contains embedded reinforcing woven glass fiber. 10.An adhesive tape as claimed in claim 4 in which the adhesive layercontains embedded polymer fiber.
 11. An adhesive tape as claimed inclaim 4 in which the adhesive layer contains embedded metal fiber. 12.An adhesive tape as claimed in claim 4 in which the core strip has aplurality of substantially thick portions alternating with portions ofreduced thickness forming recesses.
 13. An adhesive tape as claimed inclaim 12 in which the thickness of the strip at said margins isincreased abruptly to substantially the thickness of said thick portionto provide fins at the margins.
 14. An adhesive tape as claimed in claim4 in which the thickness of the core strip at said margins is increasedabruptly to substantially the thickness of said thick portion to providefins at the margins.
 15. An adhesive tape as claimed in claim 14 inwhich each margin of the strip is provided with two thin fins projectingoutward from opposite faces of the strip to form recesses, at least someof the adhesive being disposed within the recesses in sufficient amountto fill the gap between the core strip and the structural parts joined.16. An adhesive tape as claimed in claim 4 having fins at said margins,the fins being curved inwardly towards the interior of the core strip,the retroflexed part of each fin being large enough to form a partiallyclosed channel the two outer surfaces of which are adapted to adhere tothe structural parts joined.
 17. An adhesive tape for joining structuralparts that comprises, in combination, a core strip having a crosssection of non-uniform thickness and having a maximum thicknessdimension much less than the width dimension of the strip acting as acarrier for an adhesive, said core strip having a Young''s modulus atleast a substantial fraction of the Young''s modulus of the structuralparts to be joined, and an adhesive layer disposed upon the both facesof the core strip, said core strip having a cross sectional shape whichprovides in the space between the joined parts a gap for adhesive thatdiminishes in thickness from at least one margin of the strip toward theinterior region thereof, thereby to provide a thin adhesive layer atsaid interior region of the core strip and a thicker adhesive layer atsaid one margin.